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  ZooKeeper-02-ZAB协议
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      <h1 id="ZooKeeper-02-ZAB协议"><a href="#ZooKeeper-02-ZAB协议" class="headerlink" title="ZooKeeper-02-ZAB协议"></a>ZooKeeper-02-ZAB协议</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><ul>
<li><code>Zab（Zookeeper Atomic Broadcast）</code>是为ZooKeeper协设计的崩溃恢复原子广播协议，它保证zookeeper集群数据的一致性和命令的全局有序性。</li>
</ul>
<a id="more"></a>



<h2 id="1-概念介绍"><a href="#1-概念介绍" class="headerlink" title="1. 概念介绍"></a>1. 概念介绍</h2><h3 id="1-1-集群角色"><a href="#1-1-集群角色" class="headerlink" title="1.1 集群角色"></a>1.1 集群角色</h3><ul>
<li><p><code>Leader</code>：同一时间集群总只允许有一个Leader，提供对客户端的读写功能，负责将数据同步至各个节点；</p>
</li>
<li><p><code>Follower</code>：提供对客户端读功能，写请求则转发给Leader处理，当Leader崩溃失联之后参与Leader选举；</p>
</li>
<li><p><code>Observer</code>：与Follower不同的是但不参与Leader选举。</p>
</li>
</ul>
<h3 id="1-2-服务状态"><a href="#1-2-服务状态" class="headerlink" title="1.2 服务状态"></a>1.2 服务状态</h3><ul>
<li><code>LOOKING</code>：当节点认为群集中没有Leader，服务器会进入LOOKING状态，目的是为了查找或者选举Leader；</li>
<li><code>FOLLOWING</code>：follower角色；</li>
<li><code>LEADING</code>：leader角色；</li>
<li><code>OBSERVING</code>：observer角色；</li>
</ul>
<p>可以知道Zookeeper是通过自身的状态来区分自己所属的角色，来执行自己应该的任务。</p>
<h3 id="1-3-ZAB四种状态"><a href="#1-3-ZAB四种状态" class="headerlink" title="1.3 ZAB四种状态"></a>1.3 ZAB四种状态</h3><ul>
<li><strong>ZAB状态</strong>Zookeeper还给ZAB定义的4种状态，反应Zookeeper从选举到对外提供服务的过程中的四个步骤。状态枚举定义：</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/153730944.png" alt="mark"></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">1. ELECTION: 集群进入选举状态，此过程会选出一个节点作为leader角色；</span><br><span class="line">2. DISCOVERY：连接上leader，响应leader心跳，并且检测leader的角色是否更改，通过此步骤之后选举出的leader才能执行真正职务；</span><br><span class="line">3. SYNCHRONIZATION：整个集群都确认leader之后，将会把leader的数据同步到各个节点，保证整个集群的数据一致性；</span><br><span class="line">4. BROADCAST：过渡到广播状态，集群开始对外提供服务。</span><br></pre></td></tr></table></figure>



<h3 id="1-4-ZXID"><a href="#1-4-ZXID" class="headerlink" title="1.4 ZXID"></a>1.4 ZXID</h3><ul>
<li><code>Zxid</code>是极为重要的概念，它是一个long型（64位）整数，分为两部分：纪元（epoch）部分和计数器（<code>counter</code>）部分，是一个<strong>全局有序</strong>的数字。<ul>
<li>epoch代表当前集群所属的哪个leader，leader的选举就类似一个朝代的更替，你前朝的剑不能斩本朝的官，用epoch代表当前命令的有效性，</li>
<li>counter是一个<strong>递增的数字</strong>。</li>
</ul>
</li>
</ul>
<h2 id="2-选举"><a href="#2-选举" class="headerlink" title="2. 选举"></a>2. 选举</h2><ul>
<li>基础概念介绍完了，下面开始介绍zab协议是怎么支持leader选举的。</li>
<li>进行leader有三个问题<ul>
<li>什么时候进行？</li>
<li>选举规则？</li>
<li>选择流程？</li>
</ul>
</li>
</ul>
<ol>
<li><p><strong>选举发生的时机</strong>Leader发生选举有两个时机，一个是服务启动的时候当整个集群都没有leader节点会进入选举状态，如果leader已经存在就会告诉该节点leader的信息，自己连接上leader，整个集群不用进入选举状态。</p>
<p>还有一个就是在服务运行中，可能会出现各种情况，服务宕机、断电、网络延迟很高的时候leader都不能再对外提供服务了，所有当其他几点通过<strong>心跳检测</strong>到leader失联之后，集群也会进入选举状态。（<strong>可能会出现脑裂问题）</strong></p>
</li>
</ol>
<ol start="2">
<li><strong>选举规则</strong>进入投票选举流程，怎么才能选举出leader？或者说按照什么规则来让其他节点都能选举你当leader。<ul>
<li>zab协议是按照几个比较规则来进行投票的筛选，如果你的票比我更好，就修改自身的投票信息，改投你当leader。</li>
<li>下面代码是zookeeper投票比较规则：</li>
<li>当其他节点的纪元比自身高投它，如果纪元相同比较自身的zxid的大小，选举zxid大的节点，这里的zxid代表节点所提交事务最大的id，zxid越大代表该节点的数据越完整。</li>
<li>最后如果epoch和zxid都相等，则比较服务的serverId，这个Id是配置zookeeper集群所配置的，所以我们配置zookeeper集群的时候可以把服务性能更高的集群的serverId配置大些，让性能好的机器担任leader角色。</li>
</ul>
</li>
</ol>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/154615159.png" alt="mark"></p>
<ol start="3">
<li><strong>选举流程</strong></li>
</ol>
<ul>
<li>时机和规则都有了，下面就是leader的选举流程：</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/154749324.jpg" alt="mark"></p>
<ul>
<li>所有节点第一票先选举自己当leader，将投票信息广播出去；</li>
<li>从队列中接受投票信息；</li>
<li>按照规则判断是否需要更改投票信息，将更改后的投票信息再次广播出去；</li>
<li>判断是否有超过一半的投票选举同一个节点，如果是选举结束根据投票结果设置自己的服务状态，选举结束，否则继续进入投票流程。</li>
</ul>
<p><strong>举个例子</strong></p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/154846036.jpg" alt="mark"></p>
<h2 id="3-广播"><a href="#3-广播" class="headerlink" title="3. 广播"></a>3. 广播</h2><ul>
<li>集群在经过leader选举之后还会有连接leader和同步两个步骤，这里就不具体分析这两个步骤的流程了，主要介绍集群<strong>对外提供服务如何保证各个节点数据的一致性</strong>。</li>
</ul>
<p>zab在广播状态中保证以下特征</p>
<ul>
<li><strong>可靠传递:</strong> 如果消息m由一台服务器传递，那么它最终将由所有服务器传递。</li>
<li><strong>全局有序:</strong> 如果一个消息a在消息b之前被一台服务器交付，那么所有服务器都交付了a和b，并且a先于b。</li>
<li><strong>因果有序:</strong> 如果消息a在因果上先于消息b并且二者都被交付，那么a必须排在b之前。</li>
</ul>
<p><strong>有序性</strong>是zab协议必须要保证的一个很重要的属性，因为zookeeper是以类似目录结构的数据结构存储数据的，必须要求命名的有序性。（比如一个命名a创建路径为/test，然后命名b创建路径为/test/123，如果不能保证有序性b命名在a之前，b命令会因为父节点不存在而创建失败。）</p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/155158176.jpg" alt="mark"></p>
<p>如上图所示，整个写请求类似一个二阶段的提交。</p>
<p>当收到客户端的写请求的时候会经历以下几个步骤：</p>
<ol>
<li>Leader收到客户端的写请求，生成一个事务（Proposal），其中包含了zxid；</li>
<li>Leader开始广播该事务，需要注意的是所有节点的通讯都是由一个FIFO的队列维护的；</li>
<li>Follower接受到事务之后，将事务写入本地磁盘，写入成功之后返回Leader一个ACK；</li>
<li>Leader收到过半的ACK之后，开始提交本事务，并广播事务提交信息</li>
<li>从节点开始提交本事务。</li>
</ol>
<p><strong>有以上流程可知，zookeeper通过二阶段提交来保证集群中数据的一致性，因为只需要收到过半的ACK就可以提交事务，所以zookeeper的数据并不是强一致性。</strong></p>
<p>zab协议的有序性保证是通过几个方面来体现的，</p>
<ul>
<li>第一，服务之前用TCP协议进行通讯，保证在网络传输中的有序性；</li>
<li>第二，节点之前都维护了一个FIFO的队列，保证全局有序性；</li>
<li>第三，通过全局递增的zxid保证因果有序性。</li>
</ul>
<h2 id="4-状态流转"><a href="#4-状态流转" class="headerlink" title="4. 状态流转"></a>4. 状态流转</h2><ul>
<li>前面介绍了zookeeper服务状态有四种，ZAB状态也有四种。这里就简单介绍一个他们之间的状态流转，更能加深对zab协议在zookeeper工作流程中的作用。</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200925/155422758.jpg" alt="mark"></p>
<ol>
<li>服务在启动或者和leader失联之后服务状态转为LOOKING；</li>
<li>如果leader不存在选举leader，如果存在直接连接leader，此时zab协议状态为ELECTION；</li>
<li>如果有超过半数的投票选择同一台server，则leader选举结束，被选举为leader的server服务状态为LEADING，其他server服务状态为FOLLOWING/OBSERVING；</li>
<li>所有server连接上leader，此时zab协议状态为DISCOVERY；</li>
<li>leader同步数据给learner，使各个从节点数据和leader保持一致，此时zab协议状态为SYNCHRONIZATION；</li>
<li>同步超过一半的server之后，集群对外提供服务，此时zab状态为BROADCAST。</li>
</ol>
<p><strong>可以知道整个zookeeper服务的工作流程类似一个状态机的转换，而zab协议就是驱动服务状态流转的关键，理解了zab就理解了zookeeper工作的关键原理</strong></p>

      
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